Research On TMCP Of Nano-scale Precipitate Strengthened Steel Which Contains Ti And Mo

The performance of steel materials are divided into two major categories of strength and toughness.In the steel material development process has always followed a principle:When wo improve the material strength,we should not damage the material toughness.In the research and development process,the best way to improve the strength of the steel material are grain refinment and precipitation strengthening.When use the way of grain refinment to develop some types of steel,the particle diameter just can reach 3-5?m;however,if we use the way of percipitation strengthening,the particle diameter can reach to a few nanometers.In order to combine to form M(C,N)compounde with the C,N and other elements which are contained in steel,the researchers use the technique of Micro-alloying to add Ti,Nb,V and other micro-alloying elements into steel,then deposited on the substrate and produce the precipitation strengthening.The method to produce Micro-alloy carbonitride precipitation strengthening can improve the strength of the material,but also maintain its toughness,so it is an effective way to improve the performance of the steel material.In such a theoretical guidance,the purpose of this design is make sure that the experiment steel has a reasonable chemical composition.A series of controlled rolling and cooling technology are used much micro-alloy carbonitrides(precipitates)can deposite on the ferrite matrix.The goal of this task is to obtain the second phase particles which are smaller than 10nm on the ferrite matrix.In order to achieve the purpose,Ti,Mo and other micro-alloying elements were added in steel.The experiment steel chemical composition was put forward based on extensive literatures,Smelting,blooming and roughing were carried out at RAL Laboratory.Northeastern University.The experiments of simulation rollying prove the possibility of the second phase which is smaller than 10nm in the ferrite matrix.(1)The thermal simulation testing was carried out on the thermal simulation machine.The cooling rates after rolling are 2 ?/s,5 ?/s,10 ?/s,20 ?/s,30?/s,40 ?/s respectively.The schedule is:heating temperature 1200 ?,finishing temperature 940 ?,two-pass compression,deformation amount 50%.The dynamic CCT curves and the static CCT curves are obtained.lt provides a theoretical basis for the actual rollying process.The transformation start temperature is about 600 ?.Observation by Metallurgical and SEM in the dynamic CCT samples,it was found that different tissue generated in the samples with the increasing cooling rate.first,there are a large number of ferrite and pearlite in the samples,and the bainite begin to gradual in the samples,finally we can find a large number of bainite and a small amount of ferrite in the samples.Combine with the CCT curve and the organization in the sample,I developed the rolling cooling rate about the controlled cooling was 10?/s.(2)The thermal simulation testing machine was carried on studying the rolling experiment.The schedule is:finishing temperature 940 ?,cooling rate 10?/s,the incubation temperatures 580?,600?,620?,640? and deformation 30%.After the TEM experiment,the precipitation behavior is found which is different from the conventional precipitation behavior on the grain boundaries,.On the ferrite matrix,there are square precipitates TiN about 30-50nm and the circular precipitates TiC about 10nm.There are two ways about the deposition way of the circular precipitates:phase deposition precipitation and dispersion precipitation.lt contains a lot of dislocation in the grain interior,and these dislocations was"bent" because of those small precipitates hinder their movement.(3)The schedule for the rolling experiment:heating temperature 1200?,open rolling temperature 1100?,the finishing temperature about800-900 ?,holding temperature about 600 ?,rolling cooling rate is 10?/s,plate 3mm thick.The highest yield strength can reach 470MPa and the highest tensile strength can reach 580MPa.The increment of yield strength on account of precipitation strengthening can reach 160-180MPa,in this case,the tissue inside the samples are ferritea small amount of bainite.Based on Yoshimass FUNAKAWA,who studied of the nanoscale(C,N)precipitates in the hot-rolled sheet and gaved the relationship between the increment of precipitation strengthening and the precipitates size.The precipitated particlestensile about 7-8nm impacted the yield strength.With TEM experiment it was found that a large number of dispersed second phase less than 10nm precipitated in the sample.It is in line with the results of the theoretical calculations.After a series of controlled rolling and cooling process,a large number of micro-alloying carbonitride secon-phase particles less than 10nm were found in the ferrite matrix.